The invention relates to a malfunction preventing circuit for a battery charger which has a two-terminal structure and which detects a voltage across charging terminals, thereby detecting the attachment of a battery, and particularly to a malfunction preventing circuit for a battery charger which prevents a malfunction due to a noise.
FIG. 2 is a block diagram showing an example of the configuration of a prior art battery charger. As illustrated, the battery charger 1 comprises: a switching regulator 11 which converts an alternating current power into a direct current power and controls the voltage and the current; a voltage detecting unit 12 which detects an output voltage; a combination of a shunt resistor 13a and a current detecting unit 13 which detects a charging current; an operation control unit 14 having a CPU (central processing unit) which outputs instructions for processing when abnormal conditions such as an abnormal voltage, an abnormal temperature rise, or the like is encountered, and when the full charge is achieved, for the on/off control of switches, etc., for the display, and the like; a display and operation unit 15 having indicator lamps, operation buttons, etc. (not shown in the figure); an output switch 16 which performs the on/off control of an output current; an attachment detecting unit 17 for detecting the attachment of a battery 2 on the basis of a voltage level; and charging terminals 18 to which the battery 2 is to be attached.
In the initial condition (i.e., in the condition where the output switch 16 is in the off state), when the battery 2 is attached to the charging terminals 18, the attachment detecting unit 17 detects the voltage of the battery 2 (e.g., 2 to 4.7 V) and outputs a signal indicative of the detection to the operation control unit 14. The operation control unit 14 then turns on the output switch 16, and causes the indicator lamp to illuminate, so that charging is started.
The switching regulator 11 has a voltage control unit 11a and a current control unit 11b. In the initial charging period, charging is performed under constant-current control, and, in the latter half of the charging period, charging is performed under constant-voltage control. The operation control unit 14 monitors the charging current. When the operation control unit detects the full charge, the output switch 16 is turned off and the indicator lamp and the like are caused to blink, so that the charging is terminated.
This example employs the method of detecting the attachment of the battery 2 in which the attachment detecting unit 17 detects the voltage across the charging terminals 18 and determines whether the battery 2 is attached or not. In other alternative attachment detecting methods, a three-terminal structure is utilized and a detection terminal dedicated to the detection of the attachment of the battery 2 is disposed, or a switch which mechanically operates in response to the attachment of the battery 2 is disposed.
In the case where the battery attachment detecting method in which the voltage across the charging terminals 18 is detected is employed, the impedance Z.sub.1 of the attachment detecting unit 17 is usually set to be high so that, when the battery charger 1 is removed from a receptacle or the like after the completion of charging so as to interrupt the AC power, the battery 2 is prevented from discharging. For example, the voltage is divided by resistors having a high resistance, and then detected.
Accordingly, the battery charger 1 may be sometimes caused to operate by external static electricity or electromagnetic noises. For example, if the charging terminals 18 are just touched with a hand in a condition where the battery 2 is not attached, the attachment detecting unit 17 may detect a noise. As a result, the operation control unit 14 turns on the output switch 16 and the switching regulator 11 operates, with the result that an abnormal voltage is generated or a short-circuit accident occurs between the charging terminals 18. Consequently, there arises a problem in that such abnormal conditions cause a failure.
In the battery charger which has the three-terminal structure in order to prevent a malfunction due to a noise from occurring, the number of terminals is increased so that the attachment structure for the battery 2 is complicated. In the detection method utilizing the switch which mechanically operates in response to the attachment of the battery 2, the mechanism for attaching the battery 2 is complicated, thereby producing a problem in that the production cost is increased.